Plasma etching method and plasma etching unit

ABSTRACT

The present invention is a plasma etching method including: an arranging step of arranging a pair of electrodes oppositely in a chamber and making one of the electrodes support a substrate to be processed in such a manner that the substrate is arranged between the electrodes, the substrate having a silicon film and an inorganic-material film adjacent to the silicon film; and an etching step of applying a high-frequency electric power to at least one of the electrodes to form a high-frequency electric field between the pair of the electrodes, supplying a process gas into the chamber to form a plasma of the process gas by means of the electric field, and selectively plasma-etching the silicon film of the substrate by means of the plasma; wherein a frequency of the high-frequency electric power applied to the at least one of the electrodes is 50 to 150 MHz in the etching step.

FIELD OF THE INVENTION

The present invention relates to a plasma etching method and a plasmaetching unit of plasma-etching a silicon film formed on a substrate tobe processed such as a semiconductor wafer, which has the silicon filmand an inorganic-material film adjacent to the silicon film.

DESCRIPTION OF THE RELATED ART

In a manufacturing step of a semiconductor device, a multilayer filmincluding a silicon film, such as a poly-silicon film, and insulatingfilms is formed on a semiconductor wafer, and then a plasma-etchingprocess is conducted in order to form a predetermined wiring pattern.

In order to conduct the plasma-etching process, various kinds of unitsare used. Among them, a capacitive-coupling type of parallel-plateplasma etching unit is used mainly. In the capacitive-coupling type ofparallel-plate plasma etching unit, a pair of parallel-plate electrodes(upper electrode and lower electrode) are arranged in a chamber, aprocess gas is introduced into the chamber, and a high-frequencyelectric power is applied to at least one of the electrodes to form ahigh-frequency electric field between the electrodes. By means of thehigh-frequency electric field, plasma of the process gas is generated sothat a plasma-etching process is conducted to a substrate to beprocessed.

In such a plasma processing unit, a high-frequency electric power of13.56 to 40 MHz is applied to the lower electrode in order to conductthe etching process.

In such conditions, for example, when a silicon film such as apoly-silicon film is etched with a mask of an inorganic-material filmsuch as an SiO₂, the etching process is conducted under a relativelyhigh pressure in order to enhance an etching selectivity with respect tothe inorganic-material film.

However, when the etching process is conducted under the conventionalrelatively high pressure, although the etching selectivity of thesilicon film with respect to the inorganic-material film is enhanced, anetching geometric control performance is not good. This problem isarisen not only in the case using the mask of the inorganic-materialfilm, but also in another case wherein an inorganic-material film isformed as a base of the silicon film.

SUMMARY OF THE INVENTION

This invention is developed by focusing the aforementioned problems inorder to resolve them effectively. An object of the present invention isto provide a plasma etching method and a plasma etching unit that canetch a silicon film adjacent to an inorganic-material film with a highetching selective ratio and a good etching geometric controlperformance.

According to a result of study by the inventors, in the etching processof the silicon film such as a poly-silicon film, plasma density isdominant, and ion energy contributes only a little. On the other hand,in the etching process of the inorganic-material film such as a SiO₂film or a SiN film, both the plasma density and the ion energy arenecessary. Thus, if the plasma density is high and the ion energy is lowto some extent, an etching selective ratio of the silicon film withrespect to the inorganic-material film can be enhanced. In the case, theion energy of the plasma indirectly corresponds to a self-bias electricvoltage of an electrode at the etching process. Thus, in order to raisethe etching selective ratio of the silicon film with respect to theinorganic-material film, finally, it is necessary to etch the siliconfilm under a condition of high plasma density and low bias.

On the other hand, in order to improve the etching geometric controlperformance, it is necessary to conduct the process under a lowpressure. However, under the above conditions, a process under a lowerpressure can achieve a high etching selective ratio. That is, if a highplasma density and a low self-bias electric voltage are achieved, theetching selective ratio of the silicon film with respect to theinorganic-material film can be enhanced under a lower pressure. Thus, ahigh etching selective ratio and a good etching geometric controlperformance may not be in conflict with each other.

According to a further result of study by the inventors, when thefrequency of the high-frequency electric power applied to the electrodeis high, a condition wherein the plasma density is high and theself-bias electric voltage is low can be generated.

The present invention is a plasma etching method comprising: anarranging step of arranging a pair of electrodes oppositely in a chamberand making one of the electrodes support a substrate to be processed insuch a manner that the substrate is arranged between the electrodes, thesubstrate having a silicon film and an inorganic-material film adjacentto the silicon film; and an etching step of applying a high-frequencyelectric power to at least one of the electrodes to form ahigh-frequency electric field between the pair of the electrodes,supplying a process gas into the chamber to form a plasma of the processgas by means of the electric field, and selectively plasma-etching thesilicon film of the substrate by means of the plasma; wherein afrequency of the high-frequency electric power applied to the at leastone of the electrodes is 50 to 150 MHz in the etching step.

According to the present invention, since the frequency of thehigh-frequency electric power applied to the electrode is 50 to 150 MHz,which is higher than prior art, even under a condition of a lowerpressure, a high plasma density and a low self-bias electric voltage canbe achieved. Thus, the silicon film can be etched with a high etchingselective ratio with respect to the inorganic-material film and with agood geometric control performance.

It is preferable that the frequency of the high-frequency electric powerapplied to the electrode is 70 to 100 MHz, in particular 100 MHz.

In addition, in the etching step, it is preferable that power density ofthe high-frequency electric power is 0.15 to 5 W/cm².

In addition, in the etching step, it is preferable that plasma densityin the chamber is 5×10⁹ to 2×10¹⁰ cm⁻³.

In addition, in the etching step, it is preferable that a pressure inthe chamber is not higher than 13.3 Pa.

In addition, the present invention is a plasma etching methodcomprising: an arranging step of arranging a pair of electrodesoppositely in a chamber and making one of the electrodes support asubstrate to be processed in such a manner that the substrate isarranged between the electrodes, the substrate having a silicon film andan inorganic-material film adjacent to the silicon film; and an etchingstep of applying a high-frequency electric power to at least one of theelectrodes to form a high-frequency electric field between the pair ofthe electrodes, supplying a process gas into the chamber to form aplasma of the process gas by means of the electric field, andselectively plasma-etching the silicon film of the substrate by means ofthe plasma; wherein in the etching step, the process gas includes atleast one of an HBr gas and a Cl2 gas, plasma density in the chamber is5×10⁹ to 2×10¹⁰ cm⁻³, and a self-bias electric voltage of the electrodesupporting the substrate to be processed is not higher than 200 V.

According to the present invention, under a condition wherein the plasmadensity in the chamber is 5×10⁹ to 2×10¹⁰ cm⁻³ and the self-biaselectric voltage of the electrode supporting the substrate to beprocessed is not higher than 200 V, plasma of the gas including at leastone of an HBr gas and a Cl2 gas is generated, so that the silicon filmcan be etched with a high etching selective ratio with respect to theinorganic-material film and with a good geometric control performance.

In the above, the inorganic-material film may comprises at least one ofa silicon oxide, a silicon nitride, a silicon oxinitride, and a siliconcarbide.

In addition, it is preferable that the high-frequency electric power isapplied to an electrode supporting the substrate to be processed. In thecase, a second high-frequency electric power of 3.2 to 13.56 MHz may beapplied to the electrode supporting the substrate to be processed, thesecond high-frequency electric power being overlapped with thehigh-frequency electric power. By overlapping the second high-frequencyelectric power of a lower frequency with the high-frequency electricpower, plasma density and ion drawing effect can be adjusted so that anetching rate of the silicon film can be raised more while a high etchingselective ratio with respect to the inorganic-material film can beassured.

It is preferable that a frequency of the second high-frequency electricpower is 13.56 MHz. If the frequency of the second high-frequencyelectric power is 13.56 MHz, it is preferable that power density of thesecond high-frequency electric power is not higher than 0.64 W/cm². Inaddition, if the second high-frequency electric power of 3.2 to 13.56MHz is applied, it is preferable that a self-bias electric voltage ofthe electrode supporting the substrate to be processed is not higherthan 200 V.

In addition, the present invention is a plasma etching unit comprising:a chamber configured to contain a substrate to be processed having asilicon film and an inorganic-material film adjacent to the siliconfilm; a pair of electrodes arranged in the chamber, one of the pair ofelectrodes being configured to support the substrate to be processed; aprocess-gas supplying system configured to supply a process gas into thechamber; a gas-discharging system configured to discharge a gas in thechamber; and a high-frequency electric power source configured to supplya high-frequency electric power for forming a plasma to at least one ofthe electrodes; wherein a frequency of the high-frequency electric powergenerated from the high-frequency electric power source is 50 to 150MHz.

It is preferable that the frequency of high-frequency electric powergenerated from the high-frequency electric power source is 70 to 100MHz, in particular 100 MHz.

Preferably, power density of the high-frequency electric power is 0.15to 5 W/cm².

In addition, it is preferable that a pressure in the chamber is nothigher than 13.3 Pa.

In addition, preferably, the high-frequency electric power is applied toan electrode supporting the substrate to be processed.

In addition, preferably, the plasma etching unit further comprises: asecond high-frequency electric power source configured to apply a secondhigh-frequency electric power of 3.2 MHz to 13.56 MHz to the electrodesupporting the substrate to be processed, the second high-frequencyelectric power being overlapped with the high-frequency electric power.In the case, preferably, a frequency of the second high-frequencyelectric power is 13.56 MHz. In addition, preferably, power density ofthe second high-frequency electric power is not higher than 0.64 W/cm².

Herein, because of the Paschen's law, an electric-discharge startingvoltage Vs takes a local minimum value (Paschen's minimum value) when aproduct pd of a gas pressure p and a distance d between the electrodestakes a certain value. The certain value of the product pd thatcorresponds to the Paschen's minimum value is smaller when the frequencyof the high-frequency electric power is higher. Thus, when the frequencyof the high-frequency electric power is high, in order to decrease theelectric-discharge starting voltage Vs to facilitate and stabilize theelectric-discharge effect, the distance d between the electrodes has tobe reduced, if the gas pressure p is constant. Thus, in the presentinvention, it is preferable that the distance between the electrodes isshorter than 50 mm. In addition, when the distance between theelectrodes is shorter than 50 mm, residence time of the gas in thechamber can be shortened. Thus, reaction products can be efficientlydischarged, and etching stop can be reduced.

In addition, it is preferable that the plasma etching unit furthercomprises a magnetic-field forming unit configured to form a magneticfield around a plasma region between the pair of electrodes.

When the frequency of the applied high-frequency electric power is high,the etching rate may be higher in a central portion as a feedingposition compared with in a peripheral portion. However, if a magneticfield is formed around a plasma region between the pair of electrodes,plasma confining effect can be achieved so that the etching rate on thesubstrate to be processed arranged in a processing space can be madesubstantially the same between in an edge portion (peripheral portion)of the substrate to be processed and in a central portion thereof. Thatis, the etching rate can be made uniform.

It is preferable that strength of the magnetic field formed around aplasma region between the pair of electrodes by the magnetic-fieldforming unit is 0.03 to 0.045 T (300 to 450 Gauss).

In addition, it is preferable that a focus ring is provided around theelectrode supporting the substrate to be processed, and that when themagnetic-field forming unit forms a magnetic field around a plasmaregion between the pair of electrodes, strength of the magnetic field onthe focus ring is not lower than 0.001 T (10 Gauss) and strength of themagnetic field on the substrate to be processed is not higher than 0.001T.

When the strength of the magnetic field on the focus ring is not lowerthan 0.001 T, drift movement of electrons may be generated on the focusring, so that the plasma density around the focus ring is raised to makethe plasma density uniform. On the other hand, when the strength of themagnetic field on the substrate to be processed is not higher than 0.001T, which substantially has no effect on the substrate to be processed,charge-up damage can be prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic vertical sectional view showing a plasma etchingunit of an embodiment according to the present invention;

FIG. 2 is a horizontal sectional view schematically showing a magneticannular unit arranged around a chamber of the plasma etching unit ofFIG. 1;

FIG. 3 is a sectional views showing a structural example ofsemiconductor wafer to which a plasma etching process according to thepresent invention is applied;

FIG. 4 is a sectional views showing another structural example ofsemiconductor wafer to which a plasma etching process according to thepresent invention is applied;

FIG. 5 is a schematic sectional view partly showing a plasma processingunit comprising a high-frequency electric power source for generatingplasma and a high-frequency electric power source for drawing ions;

FIG. 6 is a graph showing relationships between the absolute value of aself-bias electric voltage |Vdc| and plasma density Ne, in respectivecases wherein the frequency of the high-frequency electric power is 40MHz or 100 MHz, when the plasma consists of argon gas;

FIG. 7A is a graph showing etching rates of a poly-silicon film at awafer position, in respective cases wherein the high-frequency electricpower is 500 W, 1000 W or 1500 W, when the frequency of thehigh-frequency electric power is 100 MHz;

FIG. 7B is a graph showing etching rates of a poly-silicon film at awafer position, in respective cases wherein the high-frequency electricpower is 500 W or 1000 W, when the frequency of the high-frequencyelectric power is 40 MHz;

FIG. 8 is a graph showing relationships between a high-frequencyelectric power and an etching rate of the poly-silicon film, inrespective cases wherein the frequency of the high-frequency electricpower is 40 MHz or 100 MHz;

FIG. 9 is a graph showing relationships between a high-frequencyelectric power and an etching rate of the SiO₂ film, in respective caseswherein the frequency of the high-frequency electric power is 40 MHz or100 MHz;

FIG. 10 is a graph showing relationships between a high-frequencyelectric power and an etching rate of the poly-silicon film andrelationships between a high-frequency electric power and a ratio (anetching rate of the poly-silicon film/an etching rate of the SiO₂ film)corresponding to an etching selective ratio, in respective cases whereinthe frequency of the high-frequency electric power is 40 MHz or 100 MHz;

FIG. 11 is a graph showing relationships between an etching rate of thepoly-silicon film and a ratio (an etching rate of the poly-siliconfilm/an etching rate of the SiO₂ film) corresponding to an etchingselective ratio, in respective cases wherein the frequency of thehigh-frequency electric power is 40 MHz or 100 MHz;

FIG. 12A is a graph showing relationships between a pressure in thechamber at the etching process and an etching rate of the poly-siliconfilm, in respective cases wherein the frequency of the high-frequencyelectric power is 40 MHz or 100 MHz;

FIG. 12B is a graph showing relationships between a pressure in thechamber at the etching process and an etching rate of the SiO₂ film, inrespective cases wherein the frequency of the high-frequency electricpower is 40 MHz or 100 MHz;

FIG. 13 is a graph showing relationships between a pressure in thechamber and a ratio (an etching rate of the poly-silicon film/an etchingrate of the SiO₂ film) corresponding to an etching selective ratio, inrespective cases wherein the frequency of the high-frequency electricpower is 40 MHz or 100 MHz;

FIG. 14 is a graph showing relationships between a pressure in thechamber and an etching rate of the poly-silicon film and relationshipsbetween a high-frequency electric power and a ratio (an etching rate ofthe poly-silicon film/an etching rate of the SiO₂ film) corresponding toan etching selective ratio, in respective cases wherein the frequency ofthe high-frequency electric power is 40 MHz or 100 MHz;

FIG. 15 is a graph showing relationships between an etching rate of thepoly-silicon film and a ratio (an etching rate of the poly-siliconfilm/an etching rate of the SiO₂ film) corresponding to an etchingselective ratio, in respective cases wherein the frequency of thehigh-frequency electric power is 40 MHz or 100 MHz;

FIG. 16 is a graph showing relationships between the absolute value of aself-bias electric voltage |Vdc| and plasma density Ne, when the plasmaconsists of a HBr gas, in respective cases wherein the high-frequencyelectric power is 500 W, 1000 W, 1500 W or 2000 W, the frequency of thehigh-frequency electric power being 100 MHz, and the secondhigh-frequency electric power is 0 W, 200 W or 600 W, the frequency ofthe second high-frequency electric power being 13.56 MHz;

FIG. 17 is a graph showing relationships between a high-frequencyelectric power and an etching rate of the poly-silicon film andrelationships between a high-frequency electric power and a ratio (anetching rate of the poly-silicon film/an etching rate of the SiO₂ film)corresponding to an etching selective ratio;

FIG. 18 is a graph showing relationships between a second high-frequencyelectric power and an etching rate of the poly-silicon film andrelationships between a second high-frequency electric power and a ratio(an etching rate of the poly-silicon film/an etching rate of the SiO₂film) corresponding to an etching selective ratio; and

FIG. 19 is a graph comparatively showing relationships between an Ar-gasflow rate and a pressure difference ΔP of a central portion of the waferand a peripheral portion thereof, in respective cases wherein anelectrode gap is 25 mm or 40 mm, wherein the Ar gas is used as a plasmagas.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the invention will now be described with reference tothe attached drawings.

FIG. 1 is a schematic sectional view showing a plasma etching unit usedfor carrying out the present invention. The etching unit of theembodiment includes a two-stage cylindrical chamber vessel 1, which hasan upper portion 1 a having a small diameter and an lower portion 1 bhaving a large diameter. The chamber vessel 1 may be hermetically madeof any material, for example aluminum.

A supporting table 2 is arranged in the chamber vessel 1 forhorizontally supporting a wafer W as a substrate to be processed. Thesupporting table 2 may be made of any material, for example aluminum.The supporting table 2 is placed on a conductive supporting stage 4 viaan insulation plate 3. A focus ring 5 is arranged on a peripheral areaof the supporting table 2. The focus ring 5 may be made of anyconductive material or any insulating material. When the diameter of thewafer W is 200 mmφ, it is preferable that the focus ring 5 is 240 to 280mmφ. The supporting table 2, the insulation plate 3, the supportingstage 4 and the focus ring 5 can be elevated by a ball-screw mechanismincluding a ball-screw 7. A driving portion for the elevation isarranged below the supporting stage 4 and is covered by a bellows 8. Thebellows 8 may be made of any material, for example stainless steel(SUS). The chamber vessel 1 is earthed. A coolant passage (not shown) isformed in the supporting table 2 in order to cool the supporting table2. A bellows cover 9 is provided around the bellows 8.

A feeding cable 12 for supplying a high-frequency electric power isconnected to a substantially central portion of the supporting table 2.The feeding cable 12 is connected to a high-frequency electric powersource 10 via a matching box 11. A high-frequency electric power of apredetermined frequency is adapted to be supplied from thehigh-frequency electric power source 10 to the supporting table 2. Ashowerhead 16 is provided above the supporting table 2 and oppositely inparallel with the supporting table 2. The showerhead 16 is also earthed.Thus, the supporting table 2 functions as a lower electrode, and theshowerhead 16 functions as an upper electrode. That is, the supportingtable 2 and the showerhead 16 form a pair of plate electrodes.

Herein, it is preferable that the distance between the electrodes is setto be shorter than 50 mm. The reason is as follows.

Because of the Paschen's law, an electric-discharge starting voltage Vstakes a local minimum value (Paschen's minimum value) when a product pdof a gas pressure p and a distance d between the electrodes takes acertain value. The certain value of the product pd that corresponds tothe Paschen's minimum value is smaller when the frequency of thehigh-frequency electric power is higher. Thus, when the frequency of thehigh-frequency electric power is high like the present embodiment, inorder to decrease the electric-discharge starting voltage Vs tofacilitate and stabilize the electric-discharge effect, the distance dbetween the electrodes has to be reduced, if the gas pressure p isconstant. Thus, it is preferable that the distance between theelectrodes is shorter than 50 mm. In addition, when the distance betweenthe electrodes is shorter than 50 mm, residence time of the gas in thechamber can be shortened. Thus, reaction products can be efficientlydischarged, and etching stop can be reduced.

However, if the distance between the electrodes is too short, pressuredistribution on the surface of the wafer W as a substrate to beprocessed (pressure difference between in a central portion and in aperipheral portion) may become large. In the case, problems such asdeterioration of etching uniformity may be generated. Independently ongas flow rate, in order to make the pressure difference smaller than0.27 Pa (2 mTorr), it is preferable that the distance between theelectrodes is not shorter than 35 mm.

An electrostatic chuck 6 is provided on an upper surface of thesupporting table 2 in order to electrostaticly stick to the wafer W. Theelectrostatic chuck 6 consists of an insulation plate 6 b and anelectrode 6 a inserted in the insulation plate 6 b. The electrode 6 a isconnected to a direct-current power source 13. Thus, when thedirect-current power source 13 supplies an electric power to theelectrode 6 a, the semiconductor wafer W may be stuck to theelectrostatic chuck 6 by coulomb force, for example.

The coolant passage not shown is formed in the supporting table 2. Thewafer W can be controlled at a predetermined temperature by circulatinga suitable coolant in the coolant passage. In order to efficientlytransmit heat of cooling from the suitable coolant to the wafer W, agas-introducing mechanism (not shown) for supplying a He gas onto areverse surface of the wafer W is provided. In addition, a baffle plate14 is provided at an outside area of the focus ring 5. The baffle plate14 is electrically connected to the chamber vessel 1 via the supportingstage 4 and the bellows 8.

The showerhead 16 facing the supporting table 2 is provided in a ceilingof the chamber vessel 1. The showerhead 16 has a large number of gasjetting holes 18 at a lower surface thereof and a gas introducingportion 16 a at an upper portion thereof. Then, an inside space 17 isformed between the gas introducing portion 16 a and the large number ofgas jetting holes 18. The gas introducing portion 16 a is connected to agas supplying pipe 15 a. The gas supplying pipe 15 a is connected to aprocess-gas supplying system 15, which can supply a process gas foretching that consists of a reaction gas and a diluent gas.

As the reaction gas, any halogen gas may be used. As the diluent gas, anAr gas, a He gas, or any other gas generally used in this field may beused.

The process gas is supplied from the process-gas supplying system 15into the space 17 of the showerhead 16 through the gas supplying pipe 15a and the gas introducing portion 16 a. Then, the process gas is jettedfrom the gas jetting holes 18 in order to etch a film formed on thewafer W.

A discharging port 19 is formed at a part of a side wall of the lowerportion 1 b of the chamber 1. The discharging port 19 is connected to agas-discharging system 20 including a vacuum pump. A pressure of aninside of the chamber vessel 1 may be reduced to a predetermined vacuumlevel by operating the vacuum pump. A transferring port for the wafer Wand a gate valve 24 for opening and closing the transferring port arearranged at an other upper part of the side wall of the lower portion 1b of the chamber vessel 1.

A magnetic annular unit 21 is concentrically arranged around the upperportion 1 a of the chamber vessel 1. Thus, a magnetic field may beformed around a processing space between the supporting table 2 and theshowerhead 16. The magnetic annular unit 21 may be caused to revolvearound a center axis thereof (along an annular peripheral edge thereof)by a revolving mechanism 25.

The magnetic annular unit 21 has a plurality of segment magnets 22 whichare supported by a holder not shown and which are arranged annularly.Each of the plurality of segment magnets 22 consists of a permanentmagnet. In the embodiment, 16 segment magnets 22 are arranged annularly(concentrically) in a multi-pole state. That is, in the magnetic annularunit 21, adjacent two segment magnets 22 are arranged in such a mannerthat their magnetic-pole directions are opposite. Thus, a magnetic lineof force is formed between the adjacent two segment magnets 22 as shownin FIG. 2, so that a magnetic field of 0.02 to 0.2 T (200 to 2000Gauss), preferably 0.03 to 0.045 T (300 to 450 Gauss), is generated onlyaround the processing space. On the other hand, in a region wherein thewafer is placed, a substantially non-magnetic field state is generated.The above strength of the magnetic field is determined because of thefollowing reasons: if the magnetic field is too strong, a fringing fieldmay be caused; and if the magnetic field is too weak, plasma confiningeffect can not be achieved. Of course, the suitable strength of themagnetic field also depends on the unit structure or the like. That is,the range of the suitable strength of the magnetic field may bedifferent for respective units.

When the above magnetic field is formed around the processing space,strength of the magnetic field on the focus ring 5 is desirably notlower than 0.001 T (10 Gauss). In the case, drift movement of electrons(E×B drift) is generated on the focus ring, so that the plasma densityaround the wafer is increased, and hence the plasma density is madeuniform. On the other hand, in view of preventing charge-up damage ofthe wafer W, strength of the magnetic field in a portion where the waferW is positioned is desirably not higher than 0.001 T (10 Gauss).

Herein, the substantially non-magnetic state in a region occupied by thewafer means a state that there is not a magnetic field affecting theetching process in the area occupied by the wafer. That is, thesubstantially non-magnetic state includes a state that there is amagnetic field not substantially affecting the wafer process.

In the state shown in FIG. 2, a magnetic field whose density is not morethan 0.42 mT (4.2 Gauss) is applied to a peripheral area of the wafer.Thus, plasma confining function can be achieved.

When a magnetic field is formed by the magnetic annular unit of themulti-pole state, wall portions of the chamber 1 corresponding to themagnetic poles (for example, portions shown by P in FIG. 2) may belocally whittled. Thus, the magnetic annular unit 21 may be caused torevolve along the peripheral direction of the chamber by the aboverevolving mechanism 25. Thus, it is avoided that the magnetic poles arelocally abutted (located) against the chamber wall, so that it isprevented that the chamber wall is locally whittled.

Each segment magnet 22 is configured to freely revolve around aperpendicular axis thereof by a segment-magnet revolving mechanism notshown. Then, when the segment magnets 22 are caused to revolve, thestate wherein the multi-pole magnetic field is substantially formed andthe state wherein the multi-pole magnetic field is not formed can beswitched. Depending on a process condition, the multi-pole magneticfield may be effective or not. Thus, when the state wherein themulti-pole magnetic field is formed and the state wherein the multi-polemagnetic field is not formed can be switched, a suitable state can beselected correspondingly to the process condition.

As the state of the magnetic field is changed depending on thearrangement of the segment magnets, when the arrangement of the segmentmagnets is changed variously, various profiles of magnetic field can beformed. Thus, it is preferable to arrange the segment magnets so as toobtain a required profile of magnetic field.

The number of the segment magnets is not limited to the above examples.The section of each segment magnet is not limited to the rectangle, butmay have any shape such as a circle, a square, a trapezoid or the like.A magnetic material forming the segment magnets 22 is also not limited,but may be any known magnetic material such as a rare-earth magneticmaterial, ferrite magnetic material, an Arnico magnetic material, or thelike.

The above plasma etching unit can be used for an etching process to apoly-silicon film adjacent to an inorganic-material film such as an SiO₂film or an SiN film. An operation for the etching process by means ofthe plasma etching unit is explained.

For example, as shown in FIG. 3, a wafer W to be etched has a structurewherein a poly-silicon film 32 is formed on a silicon substrate 31 andwherein an inorganic-material film 33 having a predetermined pattern asa hard mask is formed on the poly-silicon film 32. Alternatively, asshown in FIG. 4, a wafer W has another structure wherein aninorganic-material film 42 consisting of SiO₂ as a gate oxide film isformed on a silicon film 41, wherein a poly-silicon film 43 as a gate isformed on the inorganic-material film 42, and wherein a resist film 44having a predetermined pattern as a mask is formed on the poly-siliconfilm 43.

The inorganic-material film 33 consists of a material generally used asa hard mask. As a suitable example, it may be a silicon oxide, a siliconnitride, a silicon oxinitride, a silicon carbide, or the like. That is,it is preferable that the inorganic-material film 43 consists of atleast one of the above materials.

In each of the above wafers W, the poly-silicon film 32 or 43 is etched.At first, the gate valve 24 is opened, the wafer W is conveyed into thechamber 1 by means of a conveying arm, and placed on the supportingtable 2. After that, the conveying arm is evacuated, the gate valve 24is closed, and the supporting table 2 is moved up to a position shown inFIG. 1. The vacuum pump of the gas-discharging system 20 creates apredetermined vacuum in the chamber 1 through the discharging port 19.

Then, a predetermined process gas, for example an HBr gas, is introducedinto the chamber 1 through the process-gas supplying system 15, forexample at a flow rate of 0.02 to 0.4 L/min (20 to 400 sccm). Thus, apressure in the chamber 1 is maintained at a predetermined pressure. Inthis state, a high-frequency electric power whose frequency is 50 to 150MHz, preferably 70 to 100 MHz, is supplied from the high-frequencyelectric power source 10 to the supporting table 2. In this case, powerper unit area i.e. power density is preferably within a range of about0.15 to about 5.0 W/cm². Then, a predetermined electric voltage isapplied from the direct current power source 13 to the electrode 6 a ofthe electrostatic chuck 6, so that the wafer W sticks to theelectrostatic chuck 6 by means of Coulomb force, for example.

When the high-frequency electric power is applied to the supportingtable 2 as the lower electrode as described above, a high-frequencyelectric field is formed in the processing space between the showerhead16 as the upper electrode and the supporting table 2 as the lowerelectrode. Thus, the process gas supplied into the processing space ismade plasma, which etches the poly-silicon film on the wafer W.

During the etching step, by means of the annular magnetic unit 21 of amulti-pole state, a magnetic field as shown in FIG. 2 can be formedaround the processing space. In the case, plasma confining effect isachieved, so that an etching rate of the wafer W may be made uniform,even in a case of a high frequency like this embodiment wherein theplasma tends to be not uniform. However, depending on the processcondition, it is preferable that the magnetic field is not formed. Inthe case, the segment magnets 22 may be caused to revolve in order toconduct the etching process under a condition wherein a magnetic fieldis substantially not formed around the processing space.

When the above magnetic field is formed, by means of the electricallyconductive or insulating focus ring 5 provided around the wafer W on thesupporting table 2, the effect of making the plasma process uniform canbe more enhanced. That is, if a plasma density at a peripheral portionof the wafer is high and an etching rate at the peripheral portion ofthe wafer is larger than that at a central portion of the wafer, byusing a focus ring made of an electrically conductive material such assilicon or SiC, even a focus-ring region functions as the lowerelectrode. Thus, a plasma-forming region is expanded over the focus ring5, the plasma process around the wafer W is promoted, so that uniformityof the etching rate is improved. In addition, if a plasma density at theperipheral portion of the wafer is low and an etching rate at theperipheral portion of the wafer is smaller than that at the centralportion of the wafer, by using a focus ring made of an electricallyinsulating material such as quartz, electric charges can not betransferred between the focus ring 5 and electrons and ions in theplasma. Thus, the plasma confining effect may be increased so thatuniformity of the etching rate is improved.

In order to adjust plasma density and ion-drawing effect, thehigh-frequency electric power for generating plasma and a secondhigh-frequency electric power for drawing ions may be overlapped witheach other. Specifically, as shown in FIG. 5, in addition to thehigh-frequency electric power source 10 for generating plasma, a secondhigh-frequency electric power source 26 for drawing ions is connected tothe matching box 11, so that they are overlapped. In the case, thefrequency of the second high-frequency electric power source 26 fordrawing ions is preferably 3.2 to 13.56 MHz, in particular 13.56 MHz.Thus, the number of parameters for controlling ion energy is increasedso that an optimum processing condition can be easily set wherein anetching rate of the poly-silicon film is raised more while a necessaryand sufficient etching selective ratio with respect to theinorganic-material film is assured.

Herein, according to a result of study by the inventors, in the etchingprocess of the poly-silicon film, the plasma density is dominant, andthe ion energy contributes only a little. On the other hand, in theetching process of the inorganic-material film, both the plasma densityand the ion energy are necessary. Thus, as shown in FIGS. 3 and 4, inthe etching process of the poly-silicon film adjacent to theinorganic-material film, in order to etch the poly-silicon film with ahigh etching selective ratio with respect to the inorganic-materialfilm, the plasma density has to be high and the ion energy has to below. That is, if the ion energy necessary for etching theinorganic-material film is low and the plasma density dominant foretching the poly-silicon film is high, only the poly-silicon film can beselectively etched. Herein, the ion energy of the plasma indirectlycorresponds to a self-bias electric voltage of an electrode at theetching process. Thus, in order to etch the poly-silicon film with ahigh etching selective ratio, finally, it is necessary to etch theorganic-material film under a condition of high plasma density and lowself-bias electric voltage.

On the other hand, in order to improve the etching geometric controlperformance, it is necessary to conduct the etching process under a lowpressure. However, when the above condition is satisfied, a processunder a lower pressure can achieve a high etching selective ratio. Thatis, if a high plasma density and a low self-bias electric voltage areachieved, the etching selective ratio of the poly-silicon film withrespect to the inorganic-material film can be enhanced even under alower pressure. Thus, a high etching selective ratio and a good etchinggeometric control performance can not be in conflict with each other.For that purpose, it was found that the frequency of the high-frequencyelectric power to be applied to the electrode is 50 to 150 MHz, which ishigher than prior art.

This is explained with reference to FIG. 6 as follows. FIG. 6 is a graphshowing relationships between the absolute value of a self-bias electricvoltage |Vdc| and plasma density Ne, in respective cases wherein thefrequency of the high-frequency electric power is 40 MHz or 100 MHz. Thetransverse axis represents the absolute value of a self-bias electricvoltage |Vdc|, and the ordinate axis represents the plasma density Ne.In the case, as the plasma gas, Ar was used for evaluation, instead ofreal etching gas. For each frequency, applied high-frequency electricpower was changed, so that values of the plasma density Ne and theabsolute value of a self-bias electric voltage |Vdc| were changed. Thatis, in the respective frequencies, if the applied high-frequencyelectric power is large, both the plasma density Ne and the absolutevalue of a self-bias electric voltage |Vdc| are large. Herein, theplasma density was measured by means of a microwave interferometer.

As shown in FIG. 6, in the case wherein the frequency of thehigh-frequency electric power is conventionally 40 MHz, when the plasmadensity is increased to enhance the etching rate of the poly-siliconfilm, the absolute value of a self-bias electric voltage |Vdc| isgreatly increased. On the other hand, in the case wherein the frequencyof the high-frequency electric power is 100 MHz that is higher thanprior art, even when the plasma density is increased, the absolute valueof a self-bias electric voltage |Vdc| is not so increased and controlledsubstantially not higher than 100 V. That is, it was found that acondition of high plasma density and low self-bias electric voltage canbe achieved. That is, if the frequency is relatively low like aconventional art, when the etching rate of the poly-silicon film isincreased in a real etching process under a low pressure, theinorganic-material film is also etched to the same extent and goodselective-etching performance is not achieved. On the other hand, if thefrequency is as high as 100 MHz, it was found that the poly-silicon filmcan be etched with a high etching selective ratio with respect to theinorganic-material film.

In addition, as seen from FIG. 6, in order to etch the poly-silicon filmunder a low pressure with a higher etching selective ratio by higherplasma density and lower self-bias electric voltage than prior art, whenthe plasma of Ar gas is formed, it may be thought preferable to form theplasma under a condition wherein the plasma density is not less than1×10¹⁰ cm⁻³ and the self-bias electric voltage of the electrode is nothigher than 100 V. Alternatively, the plasma density is not less than5×10¹⁰ cm⁻³ and the self-bias electric voltage of the electrode is nothigher than 200 V. Then, in order to satisfy such a plasma condition, itmay be estimated that the frequency of the high-frequency electric powerhas to be 50 MHz or higher.

Thus, the frequency of the high-frequency electric power for generatingplasma is set not less than 50 MHz, as described above. However, if thefrequency of the high-frequency electric power for generating plasma ishigher than 150 MHz, the uniformity of the plasma may be deteriorated.Thus, it is preferable that the frequency of the high-frequency electricpower for generating plasma is not higher than 150 MHz. In particular,in order to effectively achieve the above effect, it is preferable thatthe frequency of the high-frequency electric power for generating plasmais 70 to 100 MHz.

It is preferable that a pressure in the chamber at the etching processis not higher than 13.3 Pa (100 mT). From a view of preventing anyconflict between the etching selective ratio of the poly-silicon filmwith respect to the inorganic-material film and the etching geometriccontrol performance, it is more preferable that a pressure in thechamber is not higher than 4 Pa (30 mT). If the etching geometriccontrol performance is thought to be more important, it is further morepreferable that a pressure in the chamber is not higher than 1.33 Pa (10mT).

Next, in order to obtain a real etching rate of an poly-silicon film andan etching selective ratio with respect to an inorganic-material film,etching experiments for whole-surface formed films of an poly-siliconfilm and an inorganic-material film (SiO₂) were conducted. The result isexplained.

Herein, a 200 mm wafer was used as the wafer W, an HBr gas: 0.2 L/min(0.02 L/min only when the pressure is 0.133 Pa) was supplied as anetching gas, the gap between the electrodes was 27 mm, and the pressurein the chamber was 4 Pa.

FIG. 7A is a graph showing etching rates of a poly-silicon film at awafer position, in respective cases wherein the high-frequency electricpower is 500 W (1.59 W/cm²), 1000 W (3.18 W/cm²) or 1500 W (4.77 W/cm²),when the frequency of the high-frequency electric power is 100 MHz. FIG.7B is a graph showing etching rates of a poly-silicon film at a waferposition, in respective cases wherein the high-frequency electric poweris 500 W (1.59 W/cm²), 1000 W (3.18 W/cm²) or 1500 W (4.77 W/cm²), whenthe frequency of the high-frequency electric power is 40 MHz. FIG. 8 isa graph showing relationships between a high-frequency electric powerand an etching rate of the poly-silicon film, in respective caseswherein the frequency of the high-frequency electric power is 40 MHz or100 MHz. FIG. 9 is a graph showing relationships between ahigh-frequency electric power and an etching rate of the SiO₂ film, inrespective cases wherein the frequency of the high-frequency electricpower is 40 MHz or 100 MHz. FIG. 10 is a graph showing relationshipsbetween a high-frequency electric power and an etching rate of thepoly-silicon film and relationships between a high-frequency electricpower and a ratio (an etching rate of the poly-silicon film/an etchingrate of the SiO₂ film) corresponding to an etching selective ratio, inrespective cases wherein the frequency of the high-frequency electricpower is 40 MHz or 100 MHz. FIG. 11 is a graph showing relationshipsbetween an etching rate of the poly-silicon film and a ratio (an etchingrate of the poly-silicon film/an etching rate of the SiO₂ film)corresponding to an etching selective ratio, in respective cases whereinthe frequency of the high-frequency electric power is 40 MHz or 100 MHz.

As seen from these drawings, the etching rate of the poly-silicon filmtends to be increased when the high-frequency electric power isincreased. However, there is no great difference between those in thecases of 40 MHz and 100 MHz. In addition, at the same gas pressure andthe same power, the etching rates of the poly-silicon film in the casesof 40 MHz and 100 MHz are at the same level, but the etching rate of theSiO₂ film in the case of 40 MHz is higher than that in the case of 100MHz. Thus, it was confirmed that the ratio corresponding to an etchingselective ratio (an etching rate of the poly-silicon film/an etchingrate of the SiO₂ film) is higher in the case of 100 MHz than in the caseof 40 MHz. That is, from the experimental result of the samples forestimation, at the pressure of 4 Pa, it was confirmed that thepossibility of etching the poly-silicon film with a high etchingselective ratio is higher in the case of 100 MHz than in the case of 40MHz. If the power of the high-frequency electric power is increased toomuch, the etching rate of the poly-silicon film is increased but theetching selective ratio is decreased, because the etching rate and theetching selective ratio of the poly-silicon film are in a tradeoffrelationship. Thus, it is preferable that the power density of thehigh-frequency electric power of 100 MHz is not higher than 5 W/cm²(about 1500 W).

On the other hand, in the case of 100 MHz, when the power density isdecreased, the etching rate of the poly-silicon film is decreased andthe etching selective ratio with respect to the SiO₂ film is improved.If a base film of a film to be etched is a gate oxide film such as aSiO₂ film, since the base film has usually a thickness of several nm,the etching rate of the SiO₂ film has to be decreased to an order of 0.1nm/min. For example, when the pressure condition is 1.33 Pa (10 mT) andthe power density is 1.5 W/cm² (about 500 W), the etching rate of thepoly-silicon film is 100 nm/min, the etching selective ratio is 70, andthe etching rate of the SiO₂ film is 1.43 nm/min. Thus, in order todecrease the etching rate of the SiO₂ film to the order of 0.1 nm/min,it is estimated that the power density has to be decreased to about 0.15to 0.3 W/cm² (about 50 to 100 W). Taking into account the above point,it is preferable that the minimum high-frequency electric power is notlower than 0.3 W/cm², in particular not lower than 0.15 W/cm² (about 50W). In view of only the etching selectivity, it is preferable that thehigh-frequency electric power is not higher than 1.5 W/cm² (about 500W).

Next, other etching processes were conducted while the flow rate of theHBr gas was changed within a range of 0.02 to 0.2 L/min, the pressure inthe chamber was changed within a range of 0.133 to 13.3 Pa, thehigh-frequency electric power was fixed to 500 W, and the otherconditions were the same as the above.

FIG. 12A is a graph showing relationships between a pressure in thechamber at the etching process and an etching rate of the poly-siliconfilm, in respective cases wherein the frequency of the high-frequencyelectric power is 40 MHz or 100 MHz. FIG. 12B is a graph showingrelationships between a pressure in the chamber at the etching processand an etching rate of the SiO₂ film, in respective cases wherein thefrequency of the high-frequency electric power is 40 MHz or 100 MHz.FIG. 13 is a graph showing relationships between a pressure in thechamber and a ratio (an etching rate of the poly-silicon film/an etchingrate of the SiO₂ film) corresponding to an etching selective ratio, inrespective cases wherein the frequency of the high-frequency electricpower is 40 MHz or 100 MHz. FIG. 14 is a graph showing relationshipsbetween a pressure in the chamber and an etching rate of thepoly-silicon film and relationships between a high-frequency electricpower and a ratio (an etching rate of the poly-silicon film/an etchingrate of the SiO₂ film) corresponding to an etching selective ratio, inrespective cases wherein the frequency of the high-frequency electricpower is 40 MHz or 100 MHz. FIG. 15 is a graph showing relationshipsbetween an etching rate of the poly-silicon film and a ratio (an etchingrate of the poly-silicon film/an etching rate of the SiO₂film)-corresponding to an etching selective ratio, in respective caseswherein the frequency of the high-frequency electric power is 40 MHz or100 MHz.

As seen from these drawings, at the same high-frequency electric powerand the same pressure in the chamber, the etching rate of thepoly-silicon film is a little higher and the etching selective ratio isalso higher in the case of 100 MHz than in the case of 40 MHz. Inaddition, as the same high-frequency electric power, a high etchingselective ratio can be achieved at a lower pressure in the case of 100MHz than in the case of 40 MHz. In addition, as shown in FIG. 15, at thesame high-frequency electric power and the same etching rate, theetching selective ratio is higher in the case of 100 MHz than in thecase of 40 MHz. That is, in the case of 100 MHz, a high etchingselective ratio can be achieved under a condition of a lower pressure,which is advantageous in the etching geometric control performance, sothat both the high etching selectivity and the good etching geometriccontrol performance can be achieved.

Regarding the effect of the pressure, in the both cases of 40 MHz and100 MHz, when the pressure is higher, the etching rate and the etchingselective ratio of the poly-silicon film are better. However, in view ofthe etching geometric control performance of the poly-silicon film, itwas confirmed that a lower pressure is preferable, specifically nothigher than 13.3 Pa.

Next, a real etching gas (HBr) was used and a high-frequency electricpower of 100 MHz was applied to measure the absolute value of aself-bias electric voltage |Vdc| and plasma density Ne. The measurementresults are explained.

FIG. 16 is a graph showing relationships between the absolute value of aself-bias electric voltage |Vdc| and plasma density Ne, when the plasmaconsists of a HBr gas and the frequency of the high-frequency electricpower is 100 MHz. The transverse axis represents the absolute value of aself-bias electric voltage |Vdc|, and the ordinate axis represents theplasma density Ne. The plasma density was measured by means of amicrowave interferometer.

Herein, the pressure in the chamber was 2.7 Pa (20 mTorr). In addition,the high-frequency electric power of 100 MHz was changed within a rangeof 500 to 2000 W so that the plasma density Ne and the absolute value ofa self-bias electric voltage |Vdc| were changed. In addition, when thehigh-frequency electric power of 100 MHz was 500 W, a secondhigh-frequency electric power of 0 W, 200 W or 600 W, whose frequencywas 13.56 MHz, was overlapped with the high-frequency electric power.

As seen from FIG. 16, in the respective frequencies, when the appliedhigh-frequency electric power is larger, both the plasma density Ne andthe absolute value of a self-bias electric voltage |Vdc| are larger.

As shown in FIG. 16, in the case of the plasma of the real etching gas,compared with the plasma of the Ar gas (see FIG. 6), the plasma densitytends to be a little lower. In addition, when the second high-frequencyelectric power of the lower frequency (13 MHz) is overlapped to increasethe power, the self-bias electric voltage tends to be increased.

In addition, as seen from FIG. 16, when the second high-frequencyelectric power is not overlapped and the plasma density is increased,the absolute value of a self-bias electric voltage |Vdc| is notincreased so much, but maintained at about 100 V or less. That is, itwas found that the high plasma density and the low self-bias electricvoltage can be achieved.

FIG. 17 is a graph showing relationships between a high-frequencyelectric power and an etching rate of the poly-silicon film andrelationships between a high-frequency electric power and a ratio (anetching rate of the poly-silicon film/an etching rate of the SiO₂ film)corresponding to an etching selective ratio, wherein the secondhigh-frequency electric power is not overlapped.

When the high-frequency electric power is increased, the etching rate ofthe poly-silicon film is increased but the selective ratio is decreased.Thus, it is preferable that the high-frequency electric power is nothigher than about 1500 W (about 4.77 W/cm²). On the other hand, when thehigh-frequency electric power is decreased, the etching rate isdecreased but the selective ratio is increased. Thus, it is preferablethat the high-frequency electric power is not lower than about 500 W(about 1.5 W/cm²).

As seen from FIGS. 16 and 17, it was confirmed that a necessary etchingrate of the poly-silicon film can be achieved while the poly-siliconfilm can be etched with a high etching selective ratio with respect tothe inorganic-material film, by means of the high frequency of 100 MHz.

In addition, as seen from FIGS. 16 and 17, it is thought preferable thatthe plasma density is 5×10⁹ to 2×10¹⁰ cm⁻³ and the self-bias electricvoltage of an electrode is not higher than 200 V, in order to etch thepoly-silicon film with a high selective ratio and a required etchingrate at a low pressure.

Herein, as a process gas, instead of the gas including an HBr, a gasincluding a Cl₂ gas may be used. In the latter case, it was confirmedthat a suitable range of the plasma density is the same as the above.

FIG. 18 is a graph showing relationships between a second high-frequencyelectric power and an etching rate of the poly-silicon film andrelationships between a second high-frequency electric power and a ratio(an etching rate of the poly-silicon film/an etching rate of the SiO₂film) corresponding to an etching selective ratio, wherein thehigh-frequency electric power is fixed to 500 W and the secondhigh-frequency electric power is overlapped with the high-frequencyelectric power.

As seen from FIGS. 16 and 18, when the second high-frequency electricpower of 13 MHz is overlapped to increase the electric power, theetching rate is increased and the self-bias electric voltage of anelectrode is also increased. When the self-bias electric voltage isincreased, the etching selective ratio tends to be decreased. However,until the self-bias electric voltage reaches 200 V, that is, the secondhigh-frequency electric power reaches about 200 W (about 0.64 W/cm²),the etching selective ratio can be maintained within an allowable range.

Thus, by increasing the overlapped second high-frequency electric power(bias electric power), the etching rate can be enhanced while theetching selective ratio can be maintained at 10 or more.

In the above experiments, the gap between the electrodes was 27 mm. Asdescribed above, if the distance between the electrodes is too small,pressure distribution (pressure difference between at a central portionand at a peripheral portion) on the surface of the wafer W, which is asubstrate to be processed, becomes so large that deterioration of theetching uniformity or the like may be generated. Thus, in practice, thedistance between the electrodes is preferably 35 to 50 mm. This isexplained with reference to FIG. 19.

FIG. 19 is a graph comparatively showing relationships between an Ar-gasflow rate and a pressure difference ΔP of a central portion of the waferand a peripheral portion thereof, in respective cases wherein theelectrode gap is 25 mm or 40 mm, wherein the Ar gas is used as a plasmagas. As shown in FIG. 20, the pressure difference ΔP is smaller when thegap is 40 mm rather than 25 mm. In addition, in the case of the gap of25 mm, when the Ar-gas flow rate is increased, the pressure differenceΔP tends to be sharply increased. When the gas flow rate is higher thanabout 0.3 L/min, it exceeds 0.27 Pa (2 mTorr) as an allowable maximumpressure difference ΔP, at which deterioration of the etching uniformityor the like may not be generated. On the other hand, in the case of thegap of 40 mm, independently on the gas flow rate, the pressuredifference is smaller than 0.27 Pa (2 mTorr). Thus, it can be expectedthat the allowable maximum pressure difference ΔP at which deteriorationof the etching uniformity or the like may not be generated is ensured,independently on the gas flow rate, if the electrode gap is not lessthan about 35 mm.

The present invention is not limited to the above embodiment but may bevariously modified. For example, in the above embodiment, the siliconfilm is the poly-silicon film. However, the silicon film may be amono-crystal silicon film, an amorphous silicon film, or any othersilicon film.

In addition, in the above embodiment, as the magnetic-field generatingmeans, the annular magnetic unit in the multi-pole state is used whereinthe plurality of segment magnets consisting of permanent magnets arearranged annularly around the chamber. However, the present invention isnot limited to this manner if a magnetic-field can be formed around theprocessing space to confine the plasma. In addition, the peripheralmagnetic field for confining the plasma may be unnecessary. That is, theetching process can be conducted under a condition wherein there is nomagnetic field. In addition, the present invention can be applied to aplasma etching process conducted in a crossed electromagnetic fieldwherein a horizontal magnetic field is applied to the processing space.

In addition, in the above embodiment, the high-frequency electric powerfor generating plasma is applied to the lower electrode, but may beapplied to the upper electrode. The layer structure of the substrate tobe processed is not limited to those shown in FIGS. 3 and 4. Inaddition, the semiconductor wafer is taken as an example of thesubstrate to be processed. However, this invention is not limitedthereto, but applicable to an etching process for a silicon film inanother type of substrate to be processed.

1. A plasma etching method comprising: an arranging step of arranging apair of electrodes oppositely in a chamber and making one of theelectrodes support a substrate to be processed in such a manner that thesubstrate is arranged between the electrodes, the substrate having asilicon film and an inorganic-material film adjacent to the siliconfilm, and an etching step of applying a high-frequency electric power toat least one of the electrodes to form a high-frequency electric fieldbetween the pair of the electrodes, supplying a process gas into thechamber to form a plasma of the process gas by means of the electricfield, and selectively plasma-etching the silicon film of the substrateby means of the plasma, wherein, in the etching step, a frequency of thehigh-frequency electric power applied to the at least one of theelectrodes is 50 to 150 MHz and a pressure in the chamber is not higherthan 13.3 Pa.
 2. A plasma etching method according to claim 1, whereinthe frequency of the high-frequency electric power applied to the atleast one of the electrodes is 100 MHz in the etching step.
 3. A plasmaetching method according to claim 1, wherein in the etching step, powerdensity of the high-frequency electric power is 0.15 to 5 W/cm².
 4. Aplasma etching method according to claim 1, wherein in the etching step,plasma density in the chamber is 5×10⁹ to 2×10¹⁰ cm⁻³.
 5. A plasmaetching method according to claim 1, wherein the inorganic-material filmcomprises at least one of a silicon oxide, a silicon nitride, a siliconoxinitride, and a silicon carbide.
 6. A plasma etching method accordingto claim 1, wherein in the etching step, the high-frequency electricpower is applied to an electrode supporting the substrate to beprocessed.
 7. A plasma etching method according to claim 6, wherein inthe etching step, a second high-frequency electric power of 3.2 MHz to13.56 MHz is applied to the electrode supporting the substrate to beprocessed, the second high-frequency electric power being overlappedwith the high-frequency electric power.
 8. A plasma etching methodcomprising: an arranging step of arranging a pair of electrodesoppositely in a chamber and making one of the electrodes support asubstrate to be processed in such a manner that the substrate isarranged between the electrodes, the substrate having a silicon film andan inorganic-material film adjacent to the silicon film, and an etchingstep of applying a high-frequency electric power to at least one of theelectrodes to form a high-frequency electric field between the pair ofthe electrodes, supplying a process gas into the chamber to form aplasma of the process gas by means of the electric field, andselectively plasma-etching the silicon film of the substrate by means ofthe plasma, wherein in the etching step, a frequency of thehigh-frequency electric power applied to the at least one of theelectrodes is 50 to 150 MHz, in the etching step, the high-frequencyelectric power is applied to an electrode supporting the substrate to beprocessed, in the etching step, a second high-frequency electric poweris applied to the electrode supporting the substrate to be processed,the second high-frequency electric power being overlapped with thehigh-frequency electric power, and a frequency of the secondhigh-frequency electric power is 13.56 MHz.
 9. A plasma etching methodaccording to claim 8, wherein power density of the second high-frequencyelectric power is not higher than 0.64 W/cm².
 10. A plasma etchingmethod according to claim 7, wherein in the etching step, a self-biaselectric voltage of the electrode supporting the substrate to beprocessed is not higher than 200 V.
 11. A plasma etching methodaccording to claim 1, wherein a distance between the pair of electrodesis shorter than 50 mm.
 12. A plasma etching method comprising: anarranging step of arranging a pair of electrodes oppositely in a chamberand making one of the electrodes support a substrate to be processed insuch a manner that the substrate is arranged between the electrodes, thesubstrate having a silicon film and an inorganic-material film adjacentto the silicon film, and an etching step of applying a high-frequencyelectric power to at least one of the electrodes to form ahigh-frequency electric field between the pair of the electrodes,supplying a process gas into the chamber to form a plasma of the processgas by means of the electric field, and selectively plasma-etching thesilicon film of the substrate by means of the plasma, wherein in theetching step, a frequency of the high-frequency electric power appliedto the at least one of the electrodes is 50 to 150 MHz, and in theetching step, a magnetic field is formed around a plasma region betweenthe pair of electrodes to achieve a plasma confining effect.
 13. Aplasma etching method according to claim 12, wherein strength of themagnetic field formed around the plasma region between the pair ofelectrodes is 0.03 to 0.045 T (300 to 450 Gauss).
 14. A plasma etchingmethod according to claim 13, wherein when the magnetic field is formedaround the plasma region between the pair of electrodes, strength of themagnetic field on a focus ring provided around the substrate to beprocessed is not lower than 0.001 T (10 Gauss) and strength of themagnetic field on the substrate to be processed is not higher than 0.001T.
 15. A plasma etching method according to claim 1, wherein the siliconfilm comprises poly-silicon.
 16. A plasma etching method comprising: anarranging step of arranging a pair of electrodes oppositely in a chamberand making one of the electrodes support a substrate to be processed insuch a manner that the substrate is arranged between the electrodes, thesubstrate having a silicon film and an inorganic-material film adjacentto the silicon film, and an etching step of applying a high-frequencyelectric power to at least one of the electrodes to form ahigh-frequency electric field between the pair of the electrodes,supplying a process gas into the chamber to form a plasma of the processgas by means of the electric field, and selectively plasma-etching thesilicon film of the substrate by means of the plasma, wherein in theetching step, the process gas includes at least one of an HBr gas and aCl₂ gas, plasma density in the chamber is 5×10⁹ to 2×10¹⁰ cm⁻³, aself-bias electric voltage of the electrode supporting the substrate tobe processed is not higher than 200 V, and a pressure in the chamber isnot higher than 13.3 Pa.
 17. A plasma-etching-condition confirmingmethod comprising: an arranging step of arranging a pair of electrodesoppositely in a chamber and making one of the electrodes support asubstrate to be processed in such a manner that the substrate isarranged between the electrodes, the substrate having a silicon film andan inorganic-material film adjacent to the silicon film, and aplasma-forming step of applying a high-frequency electric power to atleast one of the electrodes to form a high-frequency electric fieldbetween the pair of the electrodes, and supplying an Ar gas into thechamber to form a plasma of the Ar gas by means of the electric field,wherein in the plasma-forming step, a confirming step is carried out toconfirm that plasma density in the chamber is not lower than 1×10¹⁰ cm⁻³and that a self-bias electric voltage of the electrode supporting thesubstrate to be processed is not higher than 100 V.
 18. A plasma etchingunit comprising: a chamber configured to contain a substrate to beprocessed having a silicon film and an inorganic-material film adjacentto the silicon film, a pair of electrodes arranged in the chamber, oneof the pair of electrodes being configured to support the substrate tobe processed, a process-gas supplying system configured to supply aprocess gas into the chamber, a gas-discharging system configured todischarge a gas in the chamber, and a high-frequency electric powersource configured to supply a high-frequency electric power for forminga plasma to at least one of the electrodes, wherein a frequency of thehigh-frequency electric power generated from the high-frequency electricpower source is 50 to 150 MHz, and a pressure in the chamber is nothigher than 13.3 Pa.
 19. A plasma etching unit according to claim 18,wherein the frequency of high-frequency electric power generated fromthe high-frequency electric power source is 100 MHz.
 20. A plasmaetching unit according to claim 18, wherein power density of thehigh-frequency electric power is 0.15 to 5 W/cm².
 21. A plasma etchingunit according to claim 18, wherein the high-frequency electric power isapplied to an electrode supporting the substrate to be processed.
 22. Aplasma etching unit according to claim 21, further comprising a secondhigh-frequency electric power source configured to apply a secondhigh-frequency electric power of 3.2 MHz to 13.56 MHz to the electrodesupporting the substrate to be processed, the second high-frequencyelectric power being overlapped with the high-frequency electric power.23. A plasma etching unit comprising: a chamber configured to contain asubstrate to be processed having a silicon film and aninorganic-material film adjacent to the silicon film, a pair ofelectrodes arranged in the chamber, one of the pair of electrodes beingconfigured to support the substrate to be processed, a process-gassupplying system configured to supply a process gas into the chamber, agas-discharging system configured to discharge a gas in the chamber, anda high-frequency electric power source configured to supply ahigh-frequency electric power for forming a plasma to at least one ofthe electrodes, wherein a frequency of the high-frequency electric powergenerated from the high-frequency electric power source is 50 to 150MHz, the high-frequency electric power is applied to an electrodesupporting the substrate to be processed, a second high-frequencyelectric power source configured to apply a second high-frequencyelectric power to the electrode supporting the substrate to be processedis provided, the second high-frequency electric power being overlappedwith the high-frequency electric power, and a frequency of the secondhigh-frequency electric power is 13.56 MHz.
 24. A plasma etching unitaccording to claim 23, wherein power density of the secondhigh-frequency electric power is not higher than 0.64 W/cm².
 25. Aplasma etching unit according to claim 18, wherein a distance betweenthe pair of electrodes is shorter than 50 mm.
 26. A plasma etching unitcomprising: a chamber configured to contain a substrate to be processedhaving a silicon film and an inorganic-material film adjacent to thesilicon film, a pair of electrodes arranged in the chamber, one of thepair of electrodes being configured to support the substrate to beprocessed, a process-gas supplying system configured to supply a processgas into the chamber, a gas-discharging system configured to discharge agas in the chamber, and a high-frequency electric power sourceconfigured to supply a high-frequency electric power for forming aplasma to at least one of the electrodes, wherein a frequency of thehigh-frequency electric power generated from the high-frequency electricpower source is 50 to 150 MHz, and a magnetic-field forming unitconfigured to form a magnetic field around a plasma region between thepair of electrodes is provided, the magnetic field achieving a plasmaconfining effect.
 27. A plasma etching unit according to claim 26,wherein strength of the magnetic field formed around the plasma regionbetween the pair of electrodes by the magnetic-field forming unit is0.03 to 0.045 T (300 to 450 Gauss).
 28. A plasma etching unit accordingto claim 27, wherein a focus ring is provided around the substrate to beprocessed, and when the magnetic-field forming unit forms the magneticfield around the plasma region between the pair of electrodes, strengthof the magnetic field on the focus ring is not lower than 0.001 T (10Gauss) and strength of the magnetic field on the substrate to beprocessed is not higher than 0.001 T.
 29. A plasma etching unitcomprising: a chamber configured to contain a substrate to be processedhaving a silicon film and an inorganic-material film adjacent to thesilicon film, a pair of electrodes arranged in the chamber, one of thepair of electrodes being configured to support the substrate to beprocessed, a process-gas supplying system configured to supply a processgas into the chamber, a gas-discharging system configured to discharge agas in the chamber, and a high-frequency electric power sourceconfigured to supply a high-frequency electric power for forming aplasma to at least one of the electrodes, wherein when a gas includingat least one of an HBr gas and a Cl₂ gas is used as the process gas,plasma density in the chamber is 5×10⁹ to 2×10¹⁰ cm⁻³, a self-biaselectric voltage of the electrode supporting the substrate to beprocessed is not higher than 200 V, and a pressure in the chamber is nothigher than 13.3 Pa.
 30. A plasma etching unit comprising: a chamberconfigured to contain a substrate to be processed having a silicon filmand an inorganic-material film adjacent to the silicon film, a pair ofelectrodes arranged in the chamber, one of the pair of electrodes beingconfigured to support the substrate to be processed, a process-gassupplying system configured to supply a process gas into the chamber, agas-discharging system configured to discharge a gas in the chamber, anda high-frequency electric power source configured to supply ahigh-frequency electric power for forming a plasma to at least one ofthe electrodes, wherein when an Ar gas is used as the process gas,plasma density in the chamber is not lower than 1×10¹⁰ cm⁻³, a self-biaselectric voltage of the electrode supporting the substrate to beprocessed is not higher than 100 V, and a pressure in the chamber is nothigher than 13.3 Pa.
 31. A plasma etching method according to claim 1,wherein in the etching step, the pressure in the chamber is not higherthan 4 Pa.
 32. A plasma etching method according to claim 1, wherein inthe etching step, the pressure in the chamber is not higher than 1.33Pa.
 33. A plasma etching unit according to claim 18, wherein thepressure in the chamber is not higher than 4 Pa.
 34. A plasma etchingunit according to claim 18, wherein the pressure in the chamber is nothigher than 1.33 Pa.